1. Field of the Invention
The present invention relates to a positive active material for a lithium-ion secondary battery, a positive electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery.
2. Description of Related Art
As a positive active material capable of providing a high energy density of a lithium-ion secondary battery, a spinel lithium nickel manganese composite oxide having an operating potential of 4.5 V or higher on the basis of metal lithium (hereinafter, a potential based on metal lithium may be represented by “vs.Li/Li+”) is known. However, in the battery using the composite oxide as the positive active material, for example, when charging and discharging are repeated under the condition of a positive electrode potential of 4.5 V (vs.Li/Li+) or higher, metal elements (typically manganese) elute from the positive active material, and the durability thereof may be significantly degraded. As a technique for a countermeasure to this problem, for example, in Materials Research Bulletin, 2008, Volume 43, Issue 12, Pages 3607-3613, enhancing durability by substituting a portion of oxygen (O) atoms in the lithium nickel manganese composite oxide with fluorine (F) atoms is described.
In a case of applying the technique to a battery in which both of a high energy density and a high output density are required (for example, an in-vehicle battery), there is still room for improvement. In a battery having a lithium nickel manganese composite oxide in which a portion of oxygen atoms is replaced with fluorine atoms, binding of the anions (O, F) and the cations (Ni, Mn) of the lithium nickel manganese composite oxide is increased due to the fluorine, and thus elution of metal elements from the composite oxide can be suppressed. This results in a tendency to increase durability. However, in a case where the amount of fluorine is small, when charging and discharging are repeated under severe conditions (for example, under the conditions of charging until the potential of the positive electrode reaches 4.5 V (vs.Li/Li+) in an environment at a high temperature of 50° C. or higher), the effect of enhancing durability cannot be sufficiently obtained, and a non-aqueous electrolyte at the positive electrode is oxidized and decomposed, resulting in an increase in the amount of generated gas. On the other hand, when the amount of fluorine is increased in consideration of the enhancement of durability and a reduction in the amount of generated gas, the interaction between the fluorine and a charge carrier (Li ions) is increased, and thus the diffusibility (mobility) of Li ions in the composite oxide is degraded. As a result, the battery resistance is increased, and particularly, input and output characteristics are degraded during high-rate charging or discharging. That is, although the increase in the amount of fluorine enhances durability or reduces the gas generation amount, there is a contradiction that the battery resistance is increased.